The goals of this project are: 1) to analyze the hematopoietic cell populations early and late after Rauscher or Moloney murine leukemia virus infection in an in vitro soft agar system in order to detect quantitatively the stage at which malignant cells appear during the course of the disease; 2) to determine the influence of helper virus pseudotype or hematopoietic growth factors on the hematopoietic target cell specificity of Abelson murine leukemia virus in vitro; 3) to screen a number of defective mammalian retroviruses for their ability to transform murine hematopoietic cells in vitro.